1. Field of the Invention
The present invention relates to a method for detecting abnormal combustion in a spark-ignition engine which is provided with a vibration sensor for detecting vibration of the engine, or an in-cylinder pressure sensor for detecting an in-cylinder pressure of the engine, and designed such that a spark timing of a spark plug is set on a retard side with respect to a compression top dead center (top dead center of a compression stroke), in a low engine speed/high engine load region and in a normal state where no abnormal combustion occurs. The present invention also relates to a spark-ignition engine using the method.
2. Description of the Background Art
Heretofore, in a spark-ignition engine equipped with a spark plug, it has been implemented to provide an ion current sensor (sensor for detecting an ion current generated by combustion of an air-fuel mixture) in a combustion chamber, and detect postignition and preignition which are likely to occur during an engine operation, based on a detection value of the ion current sensor, as disclosed, for example, in JP 2006-46140A (hereinafter referred to as “Patent Document 1”).
According to the Patent Document 1, the postignition is defined as a phenomenon that an air-fuel mixture is ignited almost without a delay with respect to a spark timing (spark ignition timing) of the spark plug. Specifically, in normal combustion, an air-fuel mixture is ignited after a given delay time (ignition delay (lag) time) from the spark timing, whereas, in the event of the postignition, an air-fuel mixture self-ignites at a timing almost without the ignition delay time. On the other hand, the preignition is defined as a phenomenon that an air-fuel mixture self-ignites before the spark timing, i.e., self-ignition occurs at a timing earlier than that in the postignition.
In other words, in the Patent Document 1, self-ignition of an air-fuel mixture occurring at a timing later than the spark timing is defined as postignition, and self-ignition of an air-fuel mixture occurring at a timing earlier than the spark timing is defined as preignition. Although the postignition is mild in terms of severity of abnormal combustion as compared with the preignition, it can be said that the postignition is a premonitory phenomenon leading to the preignition. Specifically, once the postignition occurs, a self-ignition timing will rapidly become earlier, and a risk of developing into the preignition will become higher.
If the postignition develops into the preignition, intense noise and vibration occur, and prolonged continuation of this phenomenon is likely to cause damage to a piston, etc. As above, the preignition is unignorable serious abnormal combustion. Therefore, it is desired to detect a combustion abnormality as early as possible to prevent the preignition from occurring.
For this purpose, in the Patent Document 1, the ion current sensor is used to firstly determine whether the postignition occurs. More specifically, a timing of a peak (peak timing) of a combustion ion current is identified based on a detection value of the ion current sensor, and the presence or absence of the occurrence of the postignition is determined based on whether the peak timing advances from a reference timing by a given amount or more. Then, when the occurrence of the postignition is ascertained, a certain control operation for suppressing the postignition (e.g., a control operation of increasing a fuel injection amount) is executed even before the postignition develops into the preignition.
According to the Patent Document 1, self-ignition occurring after the spark timing is defined as “postignition”, and self-ignition occurring before the spark timing is defined as “preignition”, as mentioned above. However, there is no difference between the postignition and the preignition, in that they are a phenomenon that an air-fuel mixture self-ignites at a premature timing with respect to a normal combustion initiation timing (a time point when a given delay time has elapsed from a spark timing). Thus, in this specification, the postignition and the preignition will hereinafter be referred to collectively as “preignition”, without distinguishing therebetween.
Meanwhile, in cases where the ion current sensor is used to detect the preignition as in the engine disclosed in the Patent Document 1, it is likely that the preignition cannot be accurately detected in some situations. For example, the Patent Document 1 discloses a technique of using the spark plug as the ion current sensor. In this case, due to impossibility to detect an ion current during a spark discharge, or influence of LC resonance noise in an electric circuit, etc., it becomes difficult to adequately detect a combustion ion current, which is likely to cause a risk of missing the preignition.
JP 2002-339780A (hereinafter referred to as “Patent Document 2”) discloses a technique of detecting the preignition using a vibration sensor for detecting vibration of an engine. More specifically, in the Patent Document 2, the vibration sensor is used to identify a vibration intensity and a vibration occurrence timing of vibration occurring in an engine body, wherein, when the vibration intensity is greater than a given allowable value, and the vibration occurrence timing is on an advance side with respect to a spark timing, it is determined that the preignition occurs.
In cases where the vibration sensor is used to detect the preignition as in the Patent Document 2, it is possible to solve the above problems (restriction on detection during a spark discharge, etc.) which are likely to occur when the ion current sensor is used to detect the preignition. In addition, the vibration sensor is advantageous in terms of cost, because it has heretofore been widely used to detect knocking.
However, in the technique of simply identifying the vibration intensity and the vibration occurrence timing using the vibration sensor as in the Patent Document 2, there is a problem that the preignition cannot be detected unless it develops into a serious level where large vibration occurs at a timing earlier than a spark timing. Specifically, in cases where vibration occurs at a timing later than a spark timing, there is a possibility that a cause for the vibration is not the preignition (phenomenon that an air-fuel mixture prematurely self-ignites) but knocking (phenomenon that an unburned air-fuel mixture self-ignites after initiation of combustion). Thus, in order to clearly determine that the preignition occurs, it is necessary to wait until the preignition develops into a serious level where vibration occurs at a timing earlier than the spark timing. This is undesirable in terms of reliability, durability, etc., of an engine.
The preignition can also be detected using an in-cylinder pressure sensor for detecting an in-cylinder pressure of an engine, in the same manner as that in the above technique using the vibration sensor. Specifically, when a high in-cylinder pressure greater than an allowable value is detected at a premature timing, it may be determined that the preignition occurs. However, in this case, there is also a situation where it is impossible to detect the preignition while distinguishing it from knocking, unless a level of the preignition becomes fairly serious.